In recent years, it has been disclosed that phospholipids, comprising polyunsaturated fatty acids such as eicosapentaenoic acid or docosahexaenoic acid (DHA), play an important role in physiology. The viscera of tuna, bonito, sardine and other fish contain an abundance of these phospholipids, and have drawn much attention as a candidate material for functional food. The viscera of fish are a byproduct rather than a main product in the seafood processing industry, and thus most are discarded. Therefore, an effective use of these viscera is beneficial from the viewpoint of waste reduction as well as of maximum resource utilization.
Since a polyunsaturated fatty acid such as eicosapentaenoic acid or docosahexaenoic acid is present in the viscera of fish as a constituent of phospholipid, it is important to develop a technique to efficiently extract highly concentrated phospholipids from the viscera of fish.
A type of phospholipid called phosphatidylserine is found to be effective in improving dementia, depression, and brain functions such as memory, as well as in strengthening an athlete's muscles. Thus its application to food and medicine is highly anticipated. Phosphatidylserine comprising docosahexaenoic acid is present in a human retina or brain. Phosphatidylethanolamine, which is also a type of phospholipid, is found to have an antioxidation effect, among others. Phosphatidylethanolamine comprising docosahexaenoic acid is also present in a human retina or brain. A special attention has been paid to phosphatidylserine comprising docosahexaenoic acid and phosphatidylethanolamine comprising docosahexaenoic acid, since the decrease in docosahexaenoic acid in the brain due to aging occurs in proportion to the decrease in phosphatidylserine and phosphatidylethanolamine.
However, the viscera of fish contain phospholipase, which is an enzyme that decomposes phospholipids. Therefore, the phospholipids in an organism are decomposed by phospholipase over time, leading to a gradual decrease in the amount of extractable phospholipids. Thus, it is desirable that phospholipids be extracted while the viscera are still fresh, preferably immediately after the viscera are collected from a store or a processing factory.
However, in reality, it is very difficult to set up a work schedule to extract phospholipids from viscera at a factory or the like immediately after collection. Usually, the collected viscera are temporarily frozen or freeze-dried to stop the activity of phospholipase, and are thawed later for processing.
Techniques to extract phospholipids from once freeze-dried viscera of fish are disclosed in Publicly Announced Patent Journal No. HEI 6 [1994]-77505 and Laid-Open Patent Journal No. HEI 8 [1996]-325192.
There is a problem, however, in the freezing or freeze-drying methods for preserving viscera. That is, the freezing or freeze-drying methods only suspend the activity of phospholipase (catabolic enzyme) present in the raw material; they do not completely eliminate the activity. Therefore, it is construed that decomposition of phospholipids progresses during the freezing process until the viscera are completely frozen. Also, during the thawing process, phospholipase is reactivated, decomposing phospholipids in the viscera over time, leading to an increase in neutrolipids and a decrease in phospholipids. Therefore, to obtain phospholipids in high concentration, quick-freezing and quick-thawing processes are needed. These techniques require high cost, a huge equipment installation area, and extensive maintenance.